Well I got the wire via amazon and like Mat said it only shows to half a tank which is OK around town as the low level light works but I will be venturing interstate next year so would like a replacement. Has any one got a fuel gauge sender for a 280TE they are willing to sell me? Preferably from an 1981 or older as apparently there are some changes made before then or so I read but now can't find the info Rather buy form members than look overseas.
Pete

Yes I see that part is good its the empty that could cause concern But the light lets me know if I'm down to 11 litres so as long as I'm close to a town it should be ok.......or not lol. I wonder if its to do with the resistor on the sender and if changing the value would work. Not having a spare I can't really check it but can you Math?

Pets, How long has the sender been faulty?
Has the instrument cluster been changed? The reason that I ask, is that there may be a difference in the fuel gauge unit itself between sedan and wagon, due to the differences in the sender between the 2 variants. You could check by trying a sender from a sedan in place of your wagon sender.

You may still be able to calibrate the gauge to read correctly.
I have never done this on a W123, but did it on my Morris Minor when I was young.
Measure the sender resistance full and empty (remove from tank, drain and then measure with float at top and bottom of travel) Then find resistors that match the full and empty values (My guess is that full is 0 ohms).
Remove the instrument panel from the dash, but plug it in to the wiring harness.
On the back of the fuel gauge there may be 1 or 2 holes for screwdriver adjustment of the gauge. By trial and error you should be able to set the full and empty readings correctly, by swapping the resistance for full and empty.
If there are no holes in the back of the temp gauge, you may need to remove that part of the cluster from the inst. panel and check if you can see an adjustment there. Possibly there will be a lever attached to the gauge spring. This will adjust the reading of the gauge.

One of these may help you get to the bottom of the issue and hopefully fix it.

Life is a journey, with problems to solve, lessons to learn, but most of all, experiences to enjoy.

The reason that I ask, is that there may be a difference in the fuel gauge unit itself between sedan and wagon, due to the differences in the sender between the 2 variants. You could check by trying a sender from a sedan in place of your wagon sender.

The reason that I ask, is that there may be a difference in the fuel gauge unit itself between sedan and wagon, due to the differences in the sender between the 2 variants. You could check by trying a sender from a sedan in place of your wagon sender.

Good thinking but can verify the gauges interchange happily.

By Interchange happily, do you mean that the fault is present with both sender units? If so, the gauge itself is faulty.
If you mean that the gauges for sedan and wagon are interchangeable then we still need to check the sender unit against a known good one from either body style.

Life is a journey, with problems to solve, lessons to learn, but most of all, experiences to enjoy.

hence the wire length is twice the length in the sedan which makes twice the resistance value. the cluster fuel gauge is the same across all w123s ive seen, coupe, sedan and wagon, petroleum and diesel.

i remember the wagon had a 320 or 330 ohm resistor in parallel with the wire. in parallel it lowers resistance so the wagon wire is difference.

also, the reserve light will light up in your case when the tank is low and the needle reads half full. This is because the needle and light run independent of each other, only link is the fuel level float. So in the short term you are ok.

I would find out what half the cross sectional area of awg 37 would be and find nichrome 80/20 in that size. or find the resistance of the wire in there now and find other wire of equal length with twice the resistance.

Tony the sender was fine until a few weeks ago. The cause was the nut on the bottom of the sender unit came off and with the fuel sloshing around the tube cut the wire. As the wire was too short to fix it I had to order some from Amazon (US) and when repaired it reads wrong due to the incorrect resistance I think. As I am not in the electronics field I was hoping someone here may be able to advise me on what resistance it may need.

Tony the sender was fine until a few weeks ago. The cause was the nut on the bottom of the sender unit came off and with the fuel sloshing around the tube cut the wire. As the wire was too short to fix it I had to order some from Amazon (US) and when repaired it reads wrong due to the incorrect resistance I think. As I am not in the electronics field I was hoping someone here may be able to advise me on what resistance it may need.

I'm doing Electrical Engineering at uni, so this is kinda my area.

If you still got some of the wire i linked you to, the sedan wire. Measure how much resistance that gives over 1 metre.

then write that value down.

then search for wire that has 2x that resistance per metre. Example: if i remember right it should be somewhere around the 100 Ohms per metre resistance, so you would need 200 Ohm per metre wire. ( measure 10 centimetres of the original wire ( no need to cut it, just place probes 10cm apart ) it should be somewhere around the 20 Ohm value of resistance to confirm my theory. )

Just a question. do you still have the original wire ?

( i have done the below, so it is possible )

if so, have you thought about rejoining it. ? takes a bit of skill and good steady hands. get a friend with tweezers on each end to hold the two broken ends together as you put a drop of solder on it to join it.
if there's a small gap, perhaps use a tiny bit of the 37 AWG wire to act as the joining piece, sure your value will be slightly out, by about 5%-10% but you would have full spread of the working gauge of the dash.

being half the cross sectional area ( approx 45%) of the 37 AWG sedan standard size, it should work out doubling the resistance as every other characteristic is the same ohms

resistance is futile.JPG

Sorry it's late,

this is the formula to use if you wish to check the numbers.

this should give you a pretty accurate gauge again :)

Mathuisella I just checked the wire as suggested and on the 200 ohm scale it reads 10.5 ohms. That's the 37 AWG but I think it is 21 ohms as I used the wrong setting I think. I'll double check later. I can't find the original wire to check that reading. I think the shop elves used it for Xmas decorations or rewiring their house. g Will changing the resistor get me around that? Oh I'm having trouble trying the formula you showed in the post as I can't work out what the p L & A represent. Yeh I know just a dumb old kraut but I try.
Peter